From the direct comparison of the distribution of eDIG in the galaxies in
our survey, with those already observed by
Lehnert & Heckman
(1995) and
Rand (1996), we
can conclude that we have reached similar and in a few cases somewhat
better
sensitivities. This is mostly true for the galaxies, which were obtained
with DFOSC at La Silla. The Calar Alto observations, however, have been
acquired through relatively wide
H filters in several cases
( ~ 168Å), so the
sensitivity is not as good as for
the La Silla observations, which were made through
H filters of
~ 62Å. The
estimated mean sensitivities of the
galaxies observed with DFOSC are of the order of a few cm-6 pc.

Typically there are some uncertainties associated with the scaling
procedure,
to be applied for the continuum subtraction. The scaling factor has been
determined from the ratio of the
H and continuum
countrates of
individual stars in the object frames. Of course, there arise uncertainties
as the actual continuum spectrum of a galaxy will be different from that of
individual stars, as the galactic continuum is the superposition of all
stars of the underlying stellar population. However, practice has shown,
that this is a good method and the uncertainties typically are of the order
of 5-10%. A careful analysis, however, is necessary in order to match
the ideal scaling factor. The determined factor from the intensity ratio of
the stars in the H and
continuum frame has been slightly changed in
a few cases where the derived scaling factor did not seem to match
perfectly, as artifacts were observed in the bulge region, which is a
good indicator
for the accuracy of the scaling process. Then the galaxy profile was
analyzed carefully, as the scaling factor was changed in very small
steps to
find the optimal (in a somewhat conservative manner) value, not to under-
and oversubtract the emission. This is a well established procedure
(for more details see e.g.,
Hoopes et al., 1999;
Rossa & Dettmar,
2000).

Alternatively, it is possible to determine the scaling factor using
continuum regions in the galaxy. This is something which would work much
better for face-on galaxies, but is somewhat more difficult for
edge-on galaxies, as here occasionally the prominent dust lane makes
it difficult to assess whether there is underlying emission, which is
strong enough to be seen after subtracting the continuum. Furthermore,
the true H
emission is only revealed after the continuum subtraction process.
Hence, knowing regions free of emission is somewhat difficult to judge,
prior to the subtraction process. However, testing this method, applied
to a few individual galaxies, have shown that there is a good agreement
between using stars or continuum light within the galaxies for the
determination of the scaling factor.